The study, which looked at three decades of US government data, found more than 1 million women may have been overdiagnosed for breast cancer, needlessly exposing them to the anguish of a breast cancer diagnosis and the ordeal of treatment.

"It's a lot of women. It's a very substantial harm," said Dr Gilbert Welch of The Dartmouth Institute for Health Policy & Clinical Practice in New Hampshire.

But proponents of mammograms have already begun poking holes in the study, by Welch and Dr Archie Bleyer of St. Charles Health System in Oregon and published in the New England Journal of Medicine. While mammograms are not perfect, the proponents say, their benefits still outweigh the risks.

Welch has made overdiagnosis a major focus of his research. In an earlier study, he concluded that as many as 1 million US men had been overdiagnosed with prostate cancer since the introduction in 1986 of the widely used PSA test.

How the study was done

Such studies contradict the deeply ingrained belief that cancer screening is always good because it saves lives, an idea that is being steadily challenged by studies examining the harms of screening.

"We're not the first to suggest this, and it has come at a very huge human cost - the cost of telling a large number of women they have cancer and treating women for cancer with chemotherapy and radiation and surgery," Welch said in a telephone interview.

In 2009, the US. Preventive Services Task Force, a government-backed advisory panel, issued new guidelines that suggested women should start routine mammograms at age 50 rather than 40, in part because the tests have such high false positive rates and the benefits in lives saved did not outweigh the worry and anguish caused by the false positive results.

That conclusion, which was based on a review of published studies, contradicted years of messages about the need for routine breast cancer screening starting at age 40 and triggered a backlash from cancer doctors, advocacy groups and lawmakers who said the tests save lives and are worth the risk of a false positive test result every now and then.

An independent panel of advisers in Britain that reviewed data from 11 studies backed up critics of the task force, saying that for every 10 000 50-year-old women invited to have a mammogram over the next 20 years, screening would prevent 43 deaths and result in 129 cases of overdiagnosed breast cancers, preventing one breast cancer death for every three overdiagnosed cases. The matter, however, is still far from settled.

What experts say about the study

Dr Len Lichtenfeld, deputy chief medical officer of the American Cancer Society, said the new study was an "interesting conversation starter" but not the final say about the value of mammography in the early detection of breast cancer.

"It points out issues that many if not all experts agree on, but the degree of the number of women have been impacted and the true impact of the negative side of mammography is something that other researchers would disagree with," he said.

In the latest study, Welch and Bleyer looked to see how well mammography was working as a screening tool.

They figured that to reduce the rate of death from cancer, screening needs to not only find more early-stage cancers; it must also reduce the number of cancers that are discovered at a late stage, when treatments are less likely to work.

Using government health and census data, the team compared breast cancer diagnosis between 1976-1978, before mammogram screening was widely used, with data in 2006-2008, when routine mammograms had become well established.

They found that the introduction of screening mammography in the United States has doubled the number of cases of early-stage breast cancer detected each year, but the rate at which women are diagnosed with late-stage cancer has only fallen by 8%.

Developing stages of the disease

"You see with mammography a dramatic increase in early-stage disease, but you don't see much compensatory decrease in late-stage disease. That means you are finding a whole lot of early cancers that were never destined to become late-stage," Welch said.

Welch said that mammogram screening is likely not doing much to catch cancers at an even earlier stage of the disease, when they are more treatable.

The researchers estimated that breast cancer was overdiagnosed in 1.3 million US women in the past 30 years. In 2008 alone, they estimated that breast cancer was overdiagnosed in more than 7000 women, accounting for 31% of all breast cancers diagnosed.

Welch said the findings cannot answer the question of whether women should get screened for breast cancer, nor does it suggest that there are no benefits, but the findings do challenge the assumption that mammograms are always a good idea.

"I can't discount that there may be a tiny fraction of women who are helped by screening, but we can say it is very small - much smaller than conventional wisdom," he said.

Others were quick to dismiss the findings, saying the study by Welch and Bleyer was fundamentally flawed because it underestimates the annual rate of breast cancer growth.

Dr Debra Monticciolo, a spokeswoman for the American College of Radiology and president of Society of Breast Imaging, said the paper was an "extensive analysis based on false assumptions." For their analysis, the researchers assumed in their "best guess" estimate that breast cancer rates would remain fairly stable over the study period, increasing at a rate of about a quarter of a percent a year.

"They are off by a factor of 4," said Monticciolo, who said the real figure is closer to 1% increase per year. "If you recognise breast cancer had been increasing for 1% per year, there is no overdiagnosis."

When patients go for screenings

Welch dismissed the criticism as a "circling of the wagons" by a specialty group that is "simply not open" to consider that there might be any problems with mammography. "It's is a lot easier to make a simple (and misleading) case for screening, than it is to try to do better. But it is not in the best interest of our patients," he said.

Dr Colin Begg of Memorial Sloan-Kettering Cancer Center of New York said he thinks the study makes a pretty good case for overdiagnosis, but he questions the authors' assertion that the benefits of mammography are small.

"We have had umpteen randomised trials out there that show there is a mortality benefit of about 15%," he said. Begg said the authors are concerned that women are being encouraged to get mammograms when it may not be in their best interest, but he said all screening tests have risks as well as benefits.

Breast cancer is the second-leading cause of cancer death in women, exceeded only by lung cancer, according to the American Cancer Society. "I'm assuming most women who get mammograms are vastly more concerned about preventing their death from breast cancer than they are about the false positives of mammography," he said.

Ask the Expert

Dr Gudgeon qualified in Birmingham, England, in 1968. She has more than 40 years experience in oncology, and in 1994 she founded her practice, Cape Breast Care, where she treats benign and malignant breast cancers.
Dr Boeddinghaus obtained her qualification at UCT Medical School in 1994 and her MRCP in London in 1998. She has worked extensively in the field of oncology and has a special interest in the hormonal management of breast cancer. She now works with Dr Gudgeon at Cape Breast Care. Read more.

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